Conventional heat insulators include glass fiber, glass wool, foamed urethane, foamed styrene, and other plastic foams. Glass fiber has heat conductivity of about 0.035 kcal/mh° C. at 25° C. As better heat insulators than such fiber and foams, vacuum heat insulators are proposed. Glass fiber and foamed styrene generally have heat conductivity of about five times or higher as compared with vacuum heat insulator.
A conventional vacuum heat insulator comprises a laminate film bag having gas barrier performance, and an insulating core contained in this laminate bag, and the laminate bag is evacuated in a vacuum state. The laminate film bag with gas barrier performance has a laminated body of plastic film and gas barrier layer. As the gas barrier layer, an aluminum foil of about 6 to 10 μm in thickness, or aluminum deposition layer placed on the surface of a plastic film is used. As the plastic film, polyethylene terephthalate or polypropylene is used. As the insulating core, fine powder of silica, urethane foam or the like is used. The gas barrier layer in the vacuum heat insulator has a function of preventing fresh air from invading into the laminate film bag by penetrating through the laminate film bag.
Such conventional vacuum heat insulator is used in heat insulator of low temperature atmosphere in refrigerator or cold box.
An example of laminate film used in a conventional vacuum heat insulator is shown in FIG. 6. In FIG. 6, a laminate film 5 has a protective layer 1, gas barrier layer, and a heat fusion layer 4. The gas barrier layer includes a base material 3, and a deposition layer, also referred to as a vacuum evaporated layer, 2 evaporated on the surface of the base material 3. A protective layer 1 is formed as the outermost layer. The heat fusion layer 4 functions to seal the laminate film in a bag form by heat fusion. As the protective layer 1, a plastic film of 15 μm thick polyamide-6 (tradename 6-Nylon) is used. The glass transition point of 6-Nylon is 50° C. As the heat fusion layer 4, a plastic film of polypropylene of 50 μm in thickness is used. As the deposition layer 2, aluminum is used. The deposition film thickness of the deposition layer is about 50 nm. As the support layer 3, polyethylene terephthalate (PET) or polypropylene of about 25 μm in thickness is used.
The conventional vacuum heat insulator having such deposition layer cannot be used at high temperature.
For example, if the vacuum heat insulator is used at high temperature exceeding 90° C., the plastic film supporting the deposition layer expands or shrinks thermally, and the deposition layer is cracked due to difference in coefficient of expansion between the plastic film and deposition material. Through this crack, the air invades into the vacuum heat insulator, and the internal pressure of the vacuum heat insulator rises. As a result, the insulating performance of the vacuum heat insulator drops. Thus, in the conventional vacuum heat insulator, when thermal stress is applied, the insulating performance of the vacuum heat insulator deteriorates.
As the gas molecule becomes higher in temperature, its kinetic energy increases in geometric series. Accordingly, at high temperature near 100° C. the thin deposition layer deteriorates due to this kinetic energy, and lowers in function of suppressing penetration of gas. As a result, the vacuum inside the vacuum heat insulator cannot be maintained, and the insulating performance of the vacuum heat insulator declines.
In the vacuum heat insulator using deposition layer such as aluminum deposition as gas barrier layer, as the protective layer adhered to the deposition base material or deposition surface side, polyethylene terephthalate (PET) is used. This PET film is poor in thermal dimensional stability, and since the deposition layer is very thin, the deposition layer is broken by thermal shrinkage or contraction of the PET. As a result, the gas barrier performance of the vacuum heat insulator is lowered, and the vacuum cannot be maintained in the vacuum heat insulator, thereby worsening the insulating performance.
On the other hand, in the conventional vacuum heat insulator having an aluminum foil, heat conduction propagating through the aluminum foil is great. Accordingly, the heat quantity conducting in the creeping direction of the vacuum heat insulator is greater than the heat quantity conducting in the sectional direction of the vacuum heat insulator. Therefore, in the composition of the aluminum foil formed on one side of the vacuum heat insulator contacting mutually with the aluminum foil formed on other side, or in the composition shorter in the mutual distance, the heat conducts from one side to the aluminum foil, and does not conduct to other side. That is, the heat does not conduct through the insulating core filling up the inside of the laminate film bag. As a result, the vacuum heat insulator may not exhibit sufficient insulating performance.
The conventional vacuum heat insulator having the aluminum foil cannot be used as the heat insulator for induction heating cooker, induction heating type rice cooker, or other induction heating device. That is, the aluminum foil itself is heated by induction heating, and the vacuum heat insulator having the aluminum foil does not function as heat insulator.
As a conventional hot insulating device, a thermos bottle is known. A conventional thermos bottle has double glass or stainless steel structure, with the intermediate space evacuated to vacuum. That is, the conventional thermos bottle is a vacuum double container. Hot water or cold water is put in this vacuum double container, and is kept warm or cool. A warming cooker is proposed by installing an inner container for heating in an outer container having a vacuum double structure. The food is put in the inner container, and the food is cooked by cooking range or the like, and when the food is heated to specified temperature, the inner container containing the food is transferred into the outer container, and is used in insulated state.
However, the vacuum double container requires a rigid container withstanding vacuum at atmospheric pressure. Accordingly, the thermos bottle having the vacuum double container is very heavy, and it is inconvenient when used as portable tool such as water bottle. Or, a thermos bottle using vacuum double container of stainless steel material cannot be heated from outside of the vacuum double container. Water is heated by other means, and the heated water is transferred into the vacuum double container. It is troublesome. The glass vacuum double container can transmit magnetic field for induction heating, and induction heating is applicable, but the glass is very fragile and easily broken. Other insulators such as glass fiber and plastic foamed material is lower in the insulating performance than the vacuum double container, and the temperature of the contained hot water drops easily.
A conventional electric water heater consists of container and heater. The electric water heat with insulating function includes a container, a heater, and an insulator placed around the container. Water is put in the container, and the heat is connected to the power source, and the water boils. The electric water heater having an insulating function has a function of keeping the hot water nearly at a specific temperature for a long time. The insulator used in the electric water heater having the insulating function includes glass wool, other inorganic insulator, or reflective type insulator making use of a metal reflector.
However, the glass wool and similar insulators are excellent in thermal durability, but are low in insulating performance. Accordingly, the conventional electric water heater using glass wool or similar insulator requires a large electric power for heat insulating purpose.